In many integrated circuits, it is necessary to convert an analog signal, such as a voltage, into a digital representation for further processing. There are several known analog-to-digital converter (ADC) architectures, including delta-sigma, flash ADC, successive approximation, R2R, and the like. One ADC architecture that has been recently developed is the voltage-controlled oscillator (VCO) ADC. A VCO ADC (also known as a ring-oscillator based ADC) receives an input voltage and converts the input voltage into a digital code. The input voltage is used to control inverting stages of the ring oscillator, and the frequency of ring oscillator varies according to the input voltage. The propagation of a signal through the stages of the ring oscillator is sampled by a sampling clock. The number of stages through which the signal propagates represents the instantaneous phase. The ring-oscillator ADC determines the oscillation frequency by measuring the change in phase over time, and represents the frequency, which is proportional to the input voltage, as a digital code.
Two ring-oscillator ADCs of this sort can be used to covert a differential input voltage into a corresponding digital code. The ring-oscillator core has a relatively simple design, but the ring-oscillator ADC requires additional processing circuits to produce the digital code. For example, the phase represented by the propagation of a logic state through the stages of the ring oscillator has to be sampled and encoded, and the phase has to be differentiated. To expand the range of the ring oscillator, it is known to add a range extending logic circuit, but then the output of the range extending logic circuit and the encoders must be combined using additional digital circuits. Moreover, the sampling clock needs to have a relatively high frequency, which requires the output to be decimated to provide the digital signal at a suitable lower frequency. The decimation operation is typically done using a cascaded-integrator-comb (CIC) filter, which is a digital filter that itself requires both differentiator and integrator stages mainly consisting of flip-flops and full adder cells. The result is that a practical implementation of the ring-oscillator ADC has significant complexity and power consumption that diminishes the attractiveness of this architecture.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.